Pneumatic thermostats are often used to control the temperature in individual zones of multi-zone temperature conditioning systems. A thermostat in each zone is typically supplied with air at a constant pressure from a remotely located compressor and pressure regulator. The distribution system for supplying the air to each thermostat may involve the installation of relatively long runs of pneumatic tubing at considerable expense.
In an alternative approach, the temperature conditioned air flowing in the distribution ducts is used as a source of pressurized air to supply individual zone thermostats. Thermostats thus supplied are appropriately referred to as "system powered", and thereby eliminate the need for a remote compressor, while greatly reducing the amount of pneumatic tubing required for installation.
In a VAV system, the flow of temperature conditioned air delivered to each zone may be regulated in response to the control signal generated by a system powered thermostat. To effect regulation of the flow, a bellows or air bag actuated damper typically is used. However, simple system powered thermostats cannot directly effect flow regulation of the temperature conditioned air. The bellows or air bag require too great a flow volume at the low pressures involved. To achieve the required flow volume, the control signal generated by such a thermostat may be amplified using an apparatus known in the art as a system powered volume amplifying relay.
Since air pressure in the distribution duct of a VAV system may vary over a rather wide range, the pressure of air supplied from the duct to such a system powered thermostat control must be regulated to an essentially constant value. The present invention represents a comparatively simple and inexpensive design for such an apparatus, which in one embodiment incorporates pressure regulation within a volume amplifying relay and in another embodiment incorporates pressure regulation within a thermostat.
The following U.S. Patents describe prior art relevant to the subject invention.
A system powered pneumatic thermostat is disclosed in U.S. Pat. No. 4,007,873 which includes means to regulate air pressure derived from a distribution duct, relative to air pressure downstream of a flow regulator in the duct. The pressure regulator is disclosed as a ball and inverted nozzle in fluid communication with a second nozzle through which air flow is controlled by operation of a bi-metallic lever. A volume amplifying apparatus is not disclosed.
U.S. Pat. No. 3,155,085 discloses a pneumatic relay adapted to regulate a variable supply pressure in response to temperature. An arrangement of multiple diaphragms and chambers regulates the air pressure applied to a nozzle through which air flow is also controlled by a bi-metallic lever.
The thermostatic control disclosed in U.S. Pat. No. 4,017,025 utilizes two separate supplies of air pressure from a distribution duct to effect temperature responsive control of flow in a variable air volume system. A bellows is adapted to bias a bleed valve to close in response to temperature, and the resulting pressure increase in the control line associated with the bleed valve biases a second bleed valve to close, thereby effecting control of flow in the duct. The control is also operative to maintain a constant volume flow in the duct with respect to changes in duct pressure, by means of flow sensors biasing the bleed valve associated with the temperature responsive bellows.
A modular flow controller disclosed in U.S. Pat. No. 3,806,027 is system powered, and by means of a stacked array of diaphragms and bleed valves, is operative to effect control of minimum and maximum flow and to provide volume amplification of a temperature responsive signal. There is no provision for supply signal pressure regulation. The temperature responsive signal results from the operation of a bi-metallic strip controlling a secondary bleed valve.
U.S. Pat. No. 3,817,452 includes a bleed valve controlled by a bi-metallic strip for effecting regulation of flow through a duct in response to temperature. Although the control is supplied air from the duct, there is no provision for regulating the pressure of this air.
The pneumatic relay disclosed in U.S. Pat. No. 3,971,396 provides a temperature responsive control signal output, but only after the pressure of the supply signal exceeds a predetermined value. A bi-metallic strip controls a bleed valve to restrict the flow of air through a separate bleed valve associated with the pressure regulating element.
Multiple chambers, diaphragms, and bleed valves are used to control air flow in U.S. Pat. No. 2,978,183. This patent discloses a system powered control operative to regulate the flow of both cold and warm air in response to temperature and in response to flow velocity.
U.S. Pat. Nos. 3,812,873 and 3,835,876 disclose a pneumatic signal limiter which operates to provide an output flow equal in pressure to a fluid control signal between adjustable upper and lower limits.
A relatively complex system powered, temperature responsive, pressure regulating control is disclosed in U.S. Pat. No. 2,828,076. The control is further operative to regulate flow in a distribution duct in order to maintain a constant volume delivery to a temperature conditioned space.